Mr Warmbier travelled to North Korea with Young Pioneer Tours. He was arrested in 2016 for trying to steal a propaganda sign and sentenced to 15 years in prison. He was returned to the US in a coma in June and died a week later.

How did the news come to light?

Koryo Tours and Young Pioneer Tours both revealed on Friday that they had been told of the upcoming ban by the Swedish embassy, which acts for the US as Washington has no diplomatic relations with Pyongyang.

Rowan Beard, of Young Pioneer Tours, told the BBC the embassy was urging all US nationals to depart immediately.

He said the embassy was trying to check on the number of US tourists left in the country.

There has been no official confirmation from the US. The state department continues to havean alert dated 9 Maystrongly warning US citizens not to travel to North Korea.

What form will the ban take?

A Young Pioneer Tours statement said: "It is expected that the ban will come into force within 30 days of July 27th.

"After the 30-day grace period any US national that travels to North Korea will have their passport invalidated by their government."

Rowan Beard said that the 30-day grace period would "give leeway for any [Americans] currently in the country as tourists or on humanitarian work".

Simon Cockerell, of Koryo Tours, said: "It remains to be seen what the exact text is, but the indication is it's just a straight up ban on Americans going."

Associated Press news agency quoted US officials as saying that Secretary of State Rex Tillerson had decided to implement a "geographical travel restriction" for North Korea, meaning the use of US passports to enter would be illegal.

How have the travel agencies reacted?

Mr Cockerell told the BBC the agency would still conduct tours and take Americans until the ban came into effect.

Media captionOtto Warmbier, fourth from right, seen shortly before his arrest

"If their country allows them to go, we will take them," he said.

Mr Cockerell added: "It's unfortunate for the industry but also for North Koreans who want to know what Americans are really like."

After the death of Mr Warmbier, the China-based Young Pioneer Tours announced it would no longer take visitors from the US to the country.

Why now?

There has been movement towards a ban for a while in the US, which increased with the Warmbier death.

In May, two congressmen introduced the North Korea Travel Control bill to cut off the foreign currency the country earns from American tourists.

The House foreign affairs subcommittee is scheduled to take up the draft legislation on 27 July but it would still have to go to the Senate. So there could be an executive order.

Apart from the treatment of Americans in North Korea, tension has been increasing over Pyongyang's nuclear programme.

Media captionTake a tour of the demilitarised zone between North and South Korea